This invention relates to a key-operable lock with a removable plug.
Key-operable locks with removable plugs are commonly used for locking desks, cabinets and the like. They are desirable because they simplify lock repairs and changes in key combinations. The locks generally comprise a fixed mounting member, a plug which is rotatable between a locked condition and an unlocked condition in the mounting member, a plug extension, a plurality of spring biased tumblers which are slideable in the plug between the locked condition and the unlocked condition, a standard key for locking and unlocking the plug and a special key for removing the plug. In the locked condition, one end portion of each of the tumblers extends outwardly from the plug to rotatively lock the plug to the mounting member. One of the tumblers is a control tumbler which blocks axial removal of the plug in the absence of the special key.
When the standard key is inserted into the plug, except for the control tumbler, the key engages the tumblers and shifts the outwardly extending ends of the tumblers inwardly to free the plug for rotation in the mounting member. The control tumbler continues to engage the mounting member to block its removal from the mounting member. When the master key is inserted in place of the standard key, the master key shifts the outwardly extending end of control tumbler inwardly to disengage the mounting member. This may or may not allow removal of the plug from the mounting member. In some cases the lock must be in the unlocked condition for removal of the plug from the mounting member.
The characteristic which distinguishes the locks of the relevant art is the relationship of the control tumbler relative to the plug in the locked and unlocked conditions.
Shinn U.S. Pat. No. 1,805,891; Falk U.S. Pat. No. 2,061,456; and Patriquin U.S. Pat. No. 4,398,405 are exemplary of the relevant art of key-operable locks with removable plugs. All rely on a single element which is movable with respect to the plug, i.e., a control tumbler to block the removal of the plug from the mounting member.
Shinn is a key-operable lock having a plug which is removable in either the locked or unlocked conditions. In the locked condition one end portion of the control tumbler extends outwardly from the plug and engages a fixed mounting member. The diametrically opposite end portion is always inside of the plug. When a special key is inserted, the outward extending end is retracted into the plug to allow removal of the plug in either the locked or unlocked conditions.
Falk is a key-operable lock having a plug which is removable only in the unlocked condition. In the locked condition, one end portion of the control tumbler extends outwardly from the plug and engages a fixed mounting member. The diametrically opposite end is always inside of the plug. When a standard key is inserted into the plug, the control tumbler is partially retracted into the plug but continues to block removal of the plug from the mounting member. When a special key is inserted, the outward extending end is further retracted to block removal of the plug in the locked condition but allows removal of the plug in the unlocked condition.
Patriquin discloses a key-operable lock having a plug which, like Falk, is removable only in the unlocked condition. In the locked condition, one end portion of the control tumbler extends outwardly from the plug and engages the mounting member. When a special key is inserted, the outwardly extending end of the control tumbler is retracted inside of the plug and the diametrically opposite end portion is projected outwardly from the plug to block removal of the plug in the locked condition but allow removal of the plug in the unlocked condition.
One deficiency of Shinn, Falk and Patriquin is that only one outwardly projecting end of the control tumbler prevents its forced removal from the plug by an unauthorized person.
Another deficiency of Shinn is that an unauthorized person can remove the cylinder from the mounting member in the unlocked condition with a lock pick.
Another deficiency of Patriquin is that the length of the control tumbler must be longer than Shinn and Falk because the end of the tumbler must extend out of the tumbler in the unlocked condition.